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APART from those creatures whose fragmentary remains have
been considered in the last chapter, and which belong to the
earliest of mammaliferous strata, the remains of Mammalia are
all referable to existing orders, In the pages which follow we
shall therefore deal with the actual representatives of living
families side by side with their extinct relatives. The existing
orders of Mammalia, together with those of their fossil allies, can be
plainly divided into two great subdivisions, or, as we shall term
them, sub-classes; the Mammalia as a whole being termed a class
of the Yertebrata comparable with the class Eeptilia, etc. If
has been usual, owing to the initiative of Professor Huxley, to
divide the Mammalia into three divisions of primary importance.
We shall adduce reasons later for not accepting this mode of
division, but that which allows of only two primary divisions.
These two divisions are (1) Prototheria and (2) Eutheria.
Whether the Multituberculata, Trituberculata, and Tricono-
donta, considered in the last chapter, are really to be distributed
among these two sub-classes is a matter upon which it is possible
to form an opinion, but not to dogmatise. The Prototheria
stand at the base of the mammalian series, and present many
likenesses to the Sauropsida; the Eutheria are the animals
which are most fully differentiated as mammals. We shall
commence with
To this group belongs the order Monotremata, and possibly
ialso the so-called Allotheria or Multituberculata.    As, however,